This invention relates to a liquid polyisocyanate composition having an improved freeze stability which contain an NCO group content of 15 to 30%, and a monomeric diphenylmethane diisocyanate content of less than 90%. These compositions comprise a blend of (A) an MDI adduct having an NCO group content of 15 to 30%, and (B) an allophanate-modified MDI having an NCO group content of 12 to 32.5%.
Diisocyanates which are liquid at room temperature have numerous advantages over solid diisocyanates. The most commercially important diisocyanates which are solid at room temperature at 4,4'-diphenyl-methane diisocyanate and 2,4'-diphenylmethane diisocyanate. Numerous patents have issued relating to the liquification of diphenylmethane diisocyanates (MDI).
One common route to liquification of MDI is through carbodiimidizations. Typical of this route are the processes described in, for example, U.S. Pat. Nos. 3,152,162, 3,384,653, 3,449,256, 3,640,966, 3,641,093, 3,701,796, 4,014,935 4,088,665, 4,154,752 and 4,177,205.
The most common technique to liquify MDI is through the reaction with various hydroxyl functional materials. The prior art has described numerous types of liquid isocyanates. These include both (1) reaction products of (i) MDI or modified MDI with (ii) hydroxyl functional materials such as are described in, for example, U.S. Pat. Nos. 3,644,457, 3,883,571, 4,229,347, 4,055,548, 4,102,833,4,332,742, 4,448,904, 4,490,301, 4,490,302, 4,539,156, 4,539,158, 4,883,909, 4,442,235 and 4,910,333, as well as (2) mixtures of (i) a reaction product of MDI or modified MDI and hydroxyl functional materials, with (ii) MDI, PMDI or modified MDI, such as are described in, for example, U.S. Pat. Nos. 4,031,026, 4,261,852, 4,321,333, 5,240,635 and 5,246,977.
U.S. Pat. No. 3,644,457 discloses organic diisocyanates and specifically, mixtures of organic diisocyanates which are liquid at room temperature. These compositions comprise the reaction product of solid 4,4'- and/or 2,4'-diphenylmethane diisocyanate with a branched aliphatic dihydroxy compound in a molar ratio of 0.1 to 0.3 mol of dihydroxy compound per mol of diisocyanate. Using the disclosed reaction conditions, allophanate formation by reaction of urethane groups with isocyanate groups does not occur in these isocyanate compositions.
U.S. Pat. Nos. 4,115,429 and 4,118,411 disclose low temperature (i.e., as low as -5.degree. C.) storage stable liquid diphenylmethane diisocyanates. These are produced by reacting diphenylmethane diisocyanates having a specified 2,4'-isomer content with propylene glycol or poly-1,2-propylene ether glycol. High levels of the 2,4'-isomer result in the desirable low temperature stability, however, these products exhibit considerable lower reactivity and also property changes in the polyurethane elastomers (i.e. lower tensile strength).
Other liquid isocyanates are described in U.S. Pat. Nos. 4,490,300, 4,490,301 and 4,490,302. U.S. Pat. No. 4,490,300 discloses reaction products of MDI with an aliphatic diol having a pendant aromatic groups, e.g., 2-methyl-2-phenyl-1,3-propanediol or phenyl-1,2-ethanediol. These liquid isocyanates are disclosed as being stable at room temperature. Other liquid isocyanates which are stable at room temperatures include those which comprise the reaction product of MDI with monoallylether of trimethylolpropane. Liquid isocyanate reaction products are also disclosed by U.S. Pat. No. 4,490,302. These comprise the reaction products of MDI with mixtures of monoalcohols, poly-1,2-propylene ether glycols and low molecular weight triols.
U.S. Pat. No. 4,738,991 discloses organic polyisocyanates which are characterized by allophanate linkages. These polyisocyanates are prepared by reacting an organic polyisocyanate include, 2,4' and 4,4'-diphenylmethane diisocyanate with poly- or a monohydric alcohol in the presence of an organometallic catalyst. Deactivation of the catalyst is achieved using a compound such as an inorganic acid, organic acid, organic chloroformate or an organic acid chloride.
U.S. Pat. No. 4,866,103 discloses a polyisocyanate composition for use in producing elastomers in a RIM process. This composition is the product of reacting an alcohol or thiol having an average functionality of from about 1.5 to about 4 and an average equivalent weight of at least about 500 with at lest 2 equivalents per hydroxyl and/or thiol equivalent of an organic polyisocyanate including the 4.,4'- and the 2,4'-isomers of diphenylmethane diisocyanate. These products are formed under conditions such that at least about 20% of the initially formed urethane or thiourethane groups are converted to allophanate and/or thioallophanate groups.
Other references relating to the preparation of allophanates containing isocyanate include GB Patent 994,890. This reference relates to the reaction of urethane isocyanates with excess diisocyanate either by heat alone or in the presence of a catalyst such as a metal carboxylate, a metal chelate or a tertiary amine, until the isocyanate content is reduced to that which is obtained theoretically when the complete reaction of the urethane groups is achieved.
U.S. Pat. No. 4,160,080 discloses a process for producing allophanate containing aliphatically and/or cycloaliphatically bound isocyanate groups in which compounds containing urethane groups are reacted with polyisocyanates having aliphatic and/or cycloaliphatic isocyanate groups, in the presence of a strong acid. The process is generally conducted at a temperature of from 90.degree. C. to 140.degree. C. for about 4 to 20 hours.
A method of preparing liquid diphenylmethane diisocyanate, disclosed by Japanese Patent Application No.1971-99176, discloses reacting diphenylmethane diisocyanate with aliphatic monovalent alcohol.
Novel, liquid diphenylmethane diisocyanates which contain allophanate linkages are also disclosed by U.S. Pat. Nos. 5,319,053 and 5,319,054. The liquid stable products of U.S. Pat. No. 5,319,053 are characterized by an NCO group content of 12 to 32.5%, and comprise the reaction product of an aliphatic alcohol and a specified mixture of isomers of diphenylmethane diisocyanate. This reference also discloses stable liquid MDI prepolymers which comprise the reaction product of the allophanate-modified MDI as described above, with an organic material containing two or more active hydrogen groups. U.S. Pat. No. 5,319,054 describes liquid allophanate modified MDI compositions which are storage stable at 25.degree. C. The diphenylmethane diisocyanate has a specific isomer distribution requiring 2-60% by weight of the 2,4'-diphenylmethane diisocyanate. The disclosed allophanate containing MDI prepolymers, although storage stable at 25.degree. C., are substantially lower in reactivity with polyols when compared to stable liquid MDI's according to the present invention. Allophanate modified MDI's will also result in products with lower tensile strength at higher 2,4'diphenylmethane diisocyanate contents.
Allophanate modified diphenylmethane diisocyanate prepolymers are described in U.S. Pat. No. 5,440,003. These products are stable liquids at 25.degree. C., and comprise the reaction product of an isomeric mixture of diphenylmethane diisocyanate and an aromatic alcohol such as phenol, with the resultant product being converted to the allophanate having an NCO group content of 12 to 32% by weight.
U.S. Pat. No. 5,663,272 discloses allophanate modified MDI which is a storage stable liquid at 25.degree. C. These compositions are prepared by reacting a monoisocyanate and an organic material having at least two hydroxyl groups and a molecular weight of 60 to 6,000 to form a urethane. The urethane is then reacted with an isomeric mixture of MDI to form an isocyanate product having an NCO group content of from 12 to 30%. This allophanate-modified MDI can be further reacted with an organic isocyanate-reactive material to form an allophanate-modified MDI prepolymer containing urethane, urea and/or biuret groups having an NCO group content of 5 to 29%. The use of monofunctional compounds in polyurethane formulations including prepolymers results in chain termination which yields lower molecular weight polyurethanes with inferior physical properties.
Polyisocyanate mixtures which are liquid at temperatures greater than 5.degree. C. are disclosed by U.S. Pat. No. 5,610,260. These polyisocyanates have an NCO content of 14.5 to 24% by weight and an allophanate group content of 7.7 to 14.5% by weight. The polyisocyanates described comprise reaction products of 4,4'-diphenylmethane diisocyanate with one or more monohydric alcohols having 4 to 16 carbon atoms at an NCO:OH equivalent ratio of 5:1 to 8.5:1, and a temperature of up to 160.degree. C. to form urethane groups, and during or subsequent to urethane formation, converting the urethane groups in the presence of a catalyst to allophanate groups. Allophanate modified liquid MDI products made by this process provide improved low temperature stability but are slower in reactivity when compared with products of the present invention and result generally in polyurethane elastomers with lower hardness.
Advantages of the present invention include low temperature stability combined with desirable reactivity and provision of excellent physical properties (i.e., combination of hardness, elongation and tensile strength) in resulting polyurethane elastomers.